1. Technical Field
The invention relates to a stress measurement sensor, which has a sensor element that operates according to the SAW principle, which is connected to a base by means of a joining material.
2. Background Information
A sensor element that operates according to the SAW principle, which is a component of a generic stress measurement sensor, refers to surface acoustic waves, abbreviated as SAW, and the sensor element thereof, which utilizes running characteristics that are dependent on external influences. Such sensor elements typically comprise a piezoelectrically active carrier crystal, on which patterns have been applied to a sensor surface, such, as in particular, transducer patterns and acoustic reflectors. Surface acoustic waves are created on the sensor surface by means of the electrical excitation of the carrier crystal, which propagate across this sensor surface at a propagation velocity. Stress sensors having such sensor elements, which operate according to the SAW principle, are formed by exploiting the effect that travel times of the surface acoustic waves between structures that are disposed on the surface of the sensor element are a function of mechanical stresses (deformations) exerted on the sensor element, a relation between the degree of corresponding stresses and a change in the propagation velocity of the surface acoustic waves may be produced as a function of said relation, and a degree of mechanical stress, or deformation, respectively, exerted thereon may thereby be extrapolated by determining the travel times, or changes thereto, respectively.
Such sensor elements that operate according to the SAW principle, as well as stress measurement sensors that are equipped with such sensor elements, are used everywhere where measurements must be taken at high temperatures or other extreme environmental conditions, or where a connection of the sensors by means of cabling is not possible or very difficult. This is because, on the one hand, sensor elements that operate according to the SAW principle are very resistant to high temperatures or other extreme environmental influences such as radiation or strong electrostatic fields for example. On the other hand, scanning may be done wirelessly, which greatly simplifies the transmission of measurement data from measurement points that are difficult to access.
The base of the stress measurement sensor is thereby frequently a separate element that is independent of the object that is to be measured, which serves as a foundation, or support, respectively, of the entire stress measurement sensor built thereon, and by means of which the stress measurement sensor may then be bound to an object that is to be measured, or fixed thereto, respectively. It is also possible, however, that the base of the stress measurement sensor itself is comprised of the same material as the object that is to be detected by means of measurement technology, in other words, that the object that is to be measured, or monitored by means of measurement technology, respectively, itself forms this base, or a section of this base, respectively, and the sensor element is applied directly to the object and fixed there through the use of the joining material. This type of application is the exception in current use.
As a rule, currently known stress measurement sensors, which are provided with sensor elements that operate according to the SAW principle, use a resin-based or other organic adhesive such as an epoxy resin for example, as a joining material between the sensor element and the base. Various shortcomings have been established in using this joining material. Thus, various chemical substances are produced even after a long period of time by the outgassing of such resin-based adhesives, for example solvents and the like contained therein, which condense on the sensor surface of the sensor element operating according to the SAW principle. However, the bonding of such foreign substances on the sensor surface, in turn, leads to a change in the running characteristics of the surface acoustic waves, thereby falsifying, or modifying, respectively, the measurement result of the sensor, and leads to measuring errors. In addition, resin-based compounds do not extend into the temperature ranges required for the use of appropriate stress measurement sensors in certain cases in terms of their temperature resistance, which may include use at 400° C. or more. In addition, resin-based, and other, organic adhesive compounds of the known type tend to age, so that the reliable service life of a stress measurement sensor formed with such a joining material is temporally limited.
A further known problem associated with a resin-based or organic joining agent is so-called “creep”. For the sensor, the result of this effect is that a stress that is exerted over an extended period of time is eventually dissipated in the joining agent, in that the joining agent expands, so that the measured value of the sensor element erroneously drifts to the value of the relaxed state under these circumstances. If a sensor designed in this manner is again released after such a prolonged stressed state, the expansion of the joining agent, which is now imprinted, thus in turn leads to a stress exerted on the sensor element, which stress is caused solely by the joining agent, although no external mechanical stress is being applied. In this state, the sensor value indicates an amount of tension, which erroneously is not equal to zero. This behavior is known as hysteresis.
Finally, in the case of known joining methods, there is frequently a problem that a portion of the stresses, or deformations, respectively, exerted on the base are not transferred into the sensor element, but instead are cushioned by a deformation of the layer of joining material, since that material is not hard enough. This also leads to a measurement error in the known stress measurement sensors, or at least to a greatly reduced stress sensitivity of the sensor, as compared to sensors having harder joining materials.